The present invention relates to a process for the preparation of mixed silica-zirconia sols and mixed oxides obtained therefrom.
Processes for the preparation of binary mixed oxides consisting of silica and a metal oxide, (for example Al.sub.2 O.sub.3, TiO.sub.2, ZrO.sub.2 and MgO) are well known; however, little information is given as to the structural homogeneity of the materials and, in particular, the distribution of the metal element (Al, Ti, Zr, Mg) in the silica structure. It is known that the traditional coprecipitation techniques of silica with the hydroxide of the metal element do not lead to the formation of mixed oxides with a homogeneous structure, since the precipitation pH of the relative hydroxides are somewhat different. This generally results in the production of products whose surface composition, measured with a surface analysis method such as, for example XPS (X-ray photoelectron spectroscopy), is different from the mass composition. These analyses generally indicate a tendency of the Si to be present on the surface of the particles as compare to the metal element.
To overcome or minimize problems relating to the structural homogeneity of mixed oxides based on silica, various technological solutions have been proposed which basically consist in controlling the reactivity of the precursors of the Si and element used in the preparation of the material. For example, in the preparation of mixed oxides SiO.sub.2 --ZrO.sub.2 starting from sodium silicate as the silicon precursor, compounds of Zr of the type Na.sub.4 Zr(C.sub.2 O.sub.4).nH.sub.2 O have been used to avoid the premature precipitation of the Zr. In other processes fluozirconic acid (H.sub.2 ZrF.sub.6) has been used combined with fluosilicic acid (H.sub.2 SiF.sub.6) to balance the reactivity of the Si and Zr precursors.
In processes for the preparation of binary mixed oxide sols based on SiO.sub.2 starting from alkoxides of Si and the metal element, the method most frequently used consists in prehydrolizing the Si alkoxide with a controlled quantity of water; the hydroxylate precursor thus formed is subsequently interacted with the metal alkoxide. In other processes the higher reactivity of the metal alkoxide compared to that of the Si alkoxide, is slowed down with the use of complexing agents such as for example diketones as in the case of mixed oxides SiO.sub.2 --TiO.sub.2.
The mixed oxide sols have been used to prepare glasses, e.g., ZrO.sub.2 -containing glasses, to prepare mixed oxides in particulate form, and to coat ceramic reinforcing fibers for incorporation into fiber reinforced ceramic matrix composites (CMC).
Ceramic matrix composites are flaw tolerant if the fiber coating promotes crack deflection and fiber pull-out at the fiber-matrix interface. Crack deflection requires that the interface material be weak relative to the fiber and the matrix. Ideally, fiber coatings should be smooth, of uniform thickness, have the correct composition and should not degrade filament tensile strength. Current CMC fiber-matrix interfaces are either carbon or boron nitride. However, oxidation is a major limitation to both these interfaces, particularly in the presence of water, and "pest" oxidation is a severe problem at intermediate temperatures. These problems have motivated research on oxide CMCs with replacements for C and BN that are stable at high temperatures.
Porous fiber-matrix interfaces are one candidate for an oxidation resistant interface. The inherent weakness of porous materials with respect to dense materials of the same phase is known. A fugitive phase, such as carbon, is required to hold the porosity open during matrix processing. After a dense matrix is introduced, the fugitive phase can be burned out. Zircon is a viable candidate for a porous interface. It is stable with common structural ceramics such as SiC, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, and mullite. It has a thermal expansion coefficient that closely matches SiC and Si.sub.3 N.sub.4. It also has an inherent resistance to sintering and coarsening because of low diffusion coefficients.
Accordingly, it is an object of the present invention to provide an improved carbon-rich zirconia-silica sol.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art from a reading of the following detailed disclosure of the invention.